A conventional interior magnet motor has employed a structure where an external shape of each one of poles of the rotor shapes like a curve of hyperbolic function in order to output greater torque and reduce the inductance. This structure is disclosed, e.g. in Unexamined Japanese Patent Publication No. 2002-10541.
This conventional structure of the rotor is shown in FIG. 8, which shows a sectional view of the rotor. Rotor iron core 53 fastens rotary shaft 52 at its center, and permanent magnets 51 are buried at its periphery. Each one of the poles is marked with “L” in FIG. 8, and its external shape “F” forms the curve of hyperbolic function, and this curve is linked to each other in series equal to the number of poles in total, thereby forming the external shape of the rotor.
A top of each one of the poles forms an arc, and lateral face 62 continued from the top forms a slope inwardly (toward the center) from the extension line of the arc. A space between top 61 and the stator (not shown) is small, so that the motor outputs greater torque; however, a space between lateral face 62 and the stator is greater, thereby producing smaller inductance. BEMF (back electromotive force) at a high rpm becomes smaller at a smaller inductance, so that the torque at the high rpm advantageously increases. The smaller inductance also saves energy proportionately.
On the other hand, there is a motor of which magnetic path is enlarged for lowering the inductance so that greater torque can be produced. Such a motor encounters greater cogging torque, and its induction voltage waveform strays off the sine-wave, so that the harmonic content increases. The rotor's external shape formed of the hyperbolic function curve causes the cogging torque to increase in response to a change of a tip-shape of the salient pole of stator's iron core, so that the harmonic content of the induction voltage increases.
FIG. 9 shows a waveform of an induction voltage of two poles, i.e. 360° in electrical angles. In FIG. 9, X-axis represents a rotary angle, and Y-axis represents an induction voltage. The solid line shows actual measured values, and the broken line shows an ideal sine-waveform. Harmonics of the values actually measured shown in FIG. 9 are analyzed, and induction voltages at respective harmonics are obtained as shown in FIG. 10. These drawings tell that the induction voltage shows distorted waveform from the ideal sine-waveform, and the harmonic content increases.